Add unit test to verify signature

Merge from widevine repo of http://go/wvgerrit/21521

On devices that use provisioning 3.0, the function
OEMCrypto_GenerateSignature will only be used for a license renewal.
This CL adds a call to OEMCrypto_GenerateSignature to the refresh key
tests.  Otherwise, there would be no coverage at all for that
function.

Change-Id: Icbd568eea3f9f256cc9b0b441f7907b316bb5b69

libwvdrmengine/oemcrypto/test/oec_session_util.h

@@ -148,6 +148,7 @@ class Session {
                         std::vector<uint8_t>* signature);
   void ClientSignMessage(const vector<uint8_t>& data,
                          std::vector<uint8_t>* signature);
+  void VerifyClientSignature(size_t data_length = 400);
   void FillKeyArray(const MessageData& data, OEMCrypto_KeyObject* key_array);
   void FillRefreshArray(OEMCrypto_KeyRefreshObject* key_array,
                         size_t key_count);